This invention relates to a method and system of processing image signals in which an image area of a lithographic printing plate of an offset printing press or the like is measured, the image area is divided into a plurality of zones, and an image signal is supplied to an image area measuring device that determines the quantity of ink supplied to each zone at the time of printing.
One example of the prior art device of this type is disclosed in U.S. Pat. No. 3,958,509 in which an image area is photographed with a television camera utilizing an image pick up tube to obtain image signals which are processed by an arithmetic operation circuit for determining the picture image.
In the prior art device, however, since a portion corresponding to 100% dot ratio and portions corresponding to other percentage of dot ratio of the printed images are processed by the same arithmetic operation, the quantity of ink supplied becomes proportional to the image area of each zone so that in zones containing portions corresponding to 100% dot ratio the quantity of ink supplied becomes deficient, whereas in zones not containing portions corresponding to 100% dot ratio the quantity of ink supplied becomes excessive, with the result that the quality of the printed matter is not high.
In the device described above it is essential to obtain image signals having accurate crest values corresponding to the reflectivity of each zone of the image.
More particularly, the reflectivity of each zone of the image is proportional to the dot ratio of a screen plate utilized in an offset printing press and it is desirable to determine the quantity of ink supplied in accordance with the dot ratio from the standpoint of reproducibility of the printed matter.
However, the transmission of image signals produced by a television camera exhibit a transient characteristic which is caused by the transmission characteristic of the transmission system so that when an image bearing surface is photographed portions of the image signal which should be built up and down sharply become distorted. Consequently, when a portion corresponding to 100% dot ratio having a sufficiently large area is photographed, a crest value having a high fidelity can be obtained. However, when a portion having 100% dot ratio (so-called solid portion), but having a small area, is photographed it is impossible to obtain a crest value of high fidelity due to the transient characteristic, thus resulting in an error in the measured value.